1. Field of the Invention
The preferred embodiments of the present invention are directed to logging-while-drilling (LWD) electromagnetic wave tools. More particularly, embodiments of the invention are directed to multiple frequency LWD resistivity tools. More particularly still, embodiments of the invention are directed to LWD resistivity tools where at least one of the transmitting antennas may operate at three or more frequencies.
2. Background of the Invention
Wireline logging tools may be used to gather information regarding downhole formations. One such wireline logging tool is an induction resistivity tool for determining resistivity of the downhole formation. Resistivity may be determined at multiple depths of investigation, where the depth of investigation in a wireline tool operating at a low frequency (e.g., near 20 kilo-Hertz) is a function of the spacing between each transmitter and receiver or receiver pair. To obtain multiple depths of investigation in a wireline tool, multiple transmitters may be mounted on the tool at spaced-apart locations from the receivers.
As technology has advanced in the exploration and recovery of hydrocarbons, it is now common to include an LWD tool capable of performing resistivity measurements as part of the bottomhole assembly (BHA) of a drillstring. LWD resistivity tools may have many similarities with their wireline counterparts, which may comprise multiple transmitters spaced apart from a receiver pair, use of electromagnetic wave propagation as an interrogating signal, and creation of logs of resistivity. However, the frequency of operation of an LWD tool may be different than that of a wireline tool. For example, many LWD tools may operate at a frequency of 2 mega-Hertz. It has been discovered in the related art that the depth of investigation of an LWD resistivity tool may also be a function of the frequency of the interrogating electromagnetic wave, when higher frequencies are used. Thus, a wireline tool and an LWD tool having the same spacing between the transmitters and receivers, because of differences in frequency, may have different depths of investigation. Some LWD tools may selectively use different frequencies to adjust the depths of investigation.
While some related art devices may dedicate a single antenna for use with each particular frequency, some related art devices may use a single antenna to transmit up to two frequencies. FIG. 1 illustrates an electrical schematic for a transmitting antenna operable or resonant at two frequencies. In particular, FIG. 1 illustrates a loop antenna 10 coupled to capacitors 12 and 14, as well as an LC circuit 16. The LC circuit 16 may be tuned such that at a first operational frequency, the LC circuit acts as a high impedance. In this situation, the inductance of the antenna 10 may interact only with the capacitance of capacitor 12, thus operating at a first resonant frequency. At a second frequency of operation, the LC circuit 16 may have a very low impedance. In this situation, the inductance of the antenna 10 may interact with the capacitance of capacitors 12 and 14, and also the capacitance of the LC circuit 16, thus operating at a second resonant frequency.
LWD tools may draw power from batteries within the tool. LWD tool designers strive to minimize power consumption so as to extend battery life. Every component in an electronic circuit, such as LC circuit 16, consumes power. Thus, LWD tool designers try to avoid use of additional circuits which consume battery power of the tool. Moreover, the space within which to run wires and place circuits in a downhole tool is limited. These factors militate against an LWD tool designer increasing functionality of transmitting antennas by adding circuits and power consuming components.